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  #31 (permalink)  
Old 03-30-2005, 11:32 AM
camaroman7d's Avatar
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Chris,
Electronics is my profession. The reason a relay is the "only" way to run any electric motor is simple. Running a long smaller gauge wire will result in a voltage drop the longer the wire and smaller the diameter the more the voltage drop. Using a relay when wired correctly will eliminate the drop. You also do not want to run long runs of "hot" wire in and out of the car if you can help it, that is just more potential for a short/fire. The last and more important note is there are very few automotive switches that can handle a constant high amperage draw. The switch will fail it is a matter of time. The is more to electricity that "voltage", current (amps) is the thing to be careful of. I am not sure what I said that "you had a problem/hard time with"? If he was running 2 gauge wire and a circuit breaker to the motors (electric) then there probably wouldn't be any worries. The thing is that is usually not the case. People tend to use the same size wire that is on whatever they are wiring. On the fuel pump it is probably 14-16 gauge, a run of wire that small to a constant draw from the front of a car to the rear will have a voltage drop and also get hot, if it gets hot enough the vinyl will melt and that's how fires start.

I still say 100% the problem has either been compounded or mis diagnosed. There is NO possible way the fans turning on would effect the mechanical fuel pump. I will agree it "could" effect the electric pump. With the pressure dropping to 2 psi, that tells me there is another problem and I think it's just a coincidence it happens about the time the fans turn on.

Another thing I would like to see tested is this (to eliminate the fans), start the car run the garden hose into the radiator, regulate water flow just enough to keep the engine cool without the fans. I am willing to bet one the engine gets to operating temperature or run whatever amount of time it takes for the fuel pressure to drop, it will happen again (even without the fans). If I'm wrong I will drive down to Bakersfield and fix it myself. The problem is not the fans and I doubt it is the fuel pump or pumps. It's either the pressure sender/gauge or some restriction in the fuel system, I agree it could be heat related.

One last point when did you set the fuel pressure? Did you do it cold? If so that could very well be your problem. You need to set the fuel pressure with the engine warm and running. If you set it cold or with the engine off, then the pressure drop is happening because once the fuel bowl/s are low the regulator is not letting enough fuel through to keep the pressure up. Once the fuel pump (electric)/and fuel heats up pressure will go down a bit. Cold fuel is dense, warm fuel is not, it is perfectly normal to see the fuel pressure go lower on hot days.

Another VERY important thing we haven't covered or mentioned. Where is this electric pump mounted? If it is above the fuel supply that can create a problem. Also you need to have a filter before the electric pump (meaning between the tank and the pump).

Royce

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  #32 (permalink)  
Old 03-30-2005, 11:30 PM
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Royce, I thought that is what you are getting at. I am a sr. engineer for a controls company and I completely understand your point. While a relay is a great way to elimate problems, calculating voltage drop and load on the wire based on its rated drops will prove alot of times that a bump in wire size will work fine. That said, I do agree that the relay is the best way, but not the only. However, it is pivotal to understand why the relay is important and how to wire it. If the relay is poorly placed or wire size is not taken into account, you are still pissing in the wind.

I actually run my fans on my truck straight off the fuse box(because I was too lazy to install a relay and run the wire)and have the pump relayed. I have two fans on one 12 guage at about an 8 foot length wire with no issues at all.

Chris
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Old 03-31-2005, 10:40 AM
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Chris,
I understand where you are coming from. If you are not going to wire the relay right it's not worth doing. I also agree with the calculating wire size and if you bump the size up enough that will help. The problem is your average Joe doesn't understand this. That still would prevent the problem with an automotive switch being able to carry the load either. Since you wired your directly to the fuse box there is no switch in the circuit to fail.

Personally I really think you are pushing it with your fans on a single 12 gauge. For short periods that should be fine but, if you ever get out and drive for any length of time you are running the risk of a slight melt down. I ran my duel fans off of three relays (I work on fault tolerant systems, so I am a little anal), that way any two of the relays could fail and I would still have my fans. Most fans need a fuse and relay rated at 30 amps, you are running two fans on a single fuse/wire. In a drag race only application you will probably never see a problem. I drive my cars long distances and they see a lot of street time, so they have to be rock soild and dead reliable.

If it works for you that's fine.

Royce
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Old 03-31-2005, 11:49 AM
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Wow, three relays for two fans.....you are a circuit junky.

I agree on my fan situation, and I will likely switch them up when I get time. It was one of those last minute, to start the engine I need to do this kindof things. It is fused at 20A which should pop before the wire melts. I was surprised myself that it has not popped yet. I just goes to show you that engineering in saftey factors can become redundant since every engineered piece of any system as a 20-50% saftey factor. They all add up to many times what is actually needed in some cases. I think cooling fans might be one of these instances.

Later,

Chris
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Old 04-01-2005, 12:47 AM
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Dropping fuel pressure problem fixed!

Hey everyone. Here is the latest update on my problem. Today I disconnected the electric fuel pump, re-adjusted the carb and timing and changed the liquid filled fuel pressure gauge to a non-liquid gauge. Started it up and watched the temp reach 180. The fan came on, the fuel pressure stayed at 6 psi and the temp reached up to 220 degrees before I shut it off.

The pressure never dropped below 6 psi. I don't know what happened but it seems to be OK now. So I want to thank EACH and EVERY one of you for your help. I was about to give completely up. So again I thank all of you guys.

Terry
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